FIG. 1 is a cross-section showing a conventional wood flooring for an under-floor heating system in which an untreated upper natural veneer layer 23 is laminated on a waterproof plywood layer 10′. As shown in FIG. 1, the conventional wood flooring for an under-floor heating system in which the untreated natural veneer layer 23 is UV-coated or impregnated in a resin. An adhesive layer 40 is interposed between the natural veneer layer 23 and the waterproof plywood layer 10′.
Wood flooring structures in which two melamine-impregnated overlay sheets are layered to both surfaces of a natural veneer were suggested. These wood floorings have a surface strength higher than conventional UV-coated wood floorings. However, they have problems that the natural texture is poor and curls are likely to occur, which negatively affects the quality of the finished products.
The respective layers constituting the conventional wood flooring for an under-floor heating system are briefly explained below with reference to FIG. 1.
The waterproof plywood layer 10′ is produced by layering 5 to 7 veneers together using a phenol or melamine resin adhesive in such a manner that the grain directions of the veneers are at right angles to each other, and pressing the laminate in a press.
The upper natural veneer layer is produced by impregnating or not impregnating a natural veneer in an impregnating resin, or impregnating the resin into the natural veneer under reduced or high pressure. The surface UV coating layer is produced by subjecting the upper natural veneer to top, intermediate or under coating 6˜10 times using a urethane acrylate paint in a process known in the art. Since the conventional wood flooring for an under-floor heating system comprises the waterproof plywood layer 10′, it exhibits excellent dimensional stability against heat and moisture.
However, since the untreated upper natural veneer has a problem of a low density, the wood flooring exhibits a surface scratch resistance as low as 0.5˜1 N and an impact resistance as low as 10˜20 cm. The scratch resistance is measured by scratching using a diamond needle after UV-coating the surface of the natural veneer, and the impact resistance is measured by dropping a metal ball weighing 225 g onto the surface of the natural veneer. There is a large possibility that the conventional wood flooring is easily impaired when laying heavyweight household appliances on the wood flooring and using them. Accordingly, the conventional wood flooring causes consumer complaints and cannot satisfy consumer's diverse needs. There is, thus, a need for wood floorings having improved physical properties.
In fact, the surface strength of the wood flooring for an under-floor heating system is measured in accordance with a common method known in the art. For example, the scratch resistance is measured by scratching the surface of a test piece cut in a predetermined size from the wood flooring for an under-floor heating system at room temperature using a diamond needle and is expressed in a 0.5 N unit. The impact resistance is measured by dropping a weight (225 g) from increasing height (by 10 cm) onto the surface of the test piece, and then the presence of damage to the test piece is visually observed.